In 3GPP (3rd Generation Partnership Project) LTE (Long Term Evolution), DFT (Discrete Fourier Transform)-spread-OFDM (Orthogonal Frequency Division Multiplexing) is adopted as an uplink access scheme. Generally, in the DFT-spread-OFDM, due to movement of a terminal, a frequency deviation of an oscillator, or the like, a carrier frequency difference (hereinafter referred as a “frequency offset”) occurs between a receiver and a transmitter. When the frequency offset occurs, a transmission characteristic of a signal deteriorates. In order to prevent the frequency offset, estimation of a frequency offset amount and frequency offset compensation are generally performed on a receiver side.
The following describes
Initial Access Procedure in the 3GPP LTE; and
General Transmission and Reception Processing; and then
Frequency Offset Estimation and Compensation and Problem of the Frequency Offset Estimation and Compensation
First, an overview of the initial access procedure will be described. In the LTE, a random access procedure is used as means for establishing synchronization of an uplink when a terminal undergoes a transition from an idle state to a connection state. FIG. 1 is a diagram illustrating the random access procedure. FIG. 1 corresponds to the figure (FIG. 10.1.5. 1-1 Contention Based Random Access Procedure) on page 53 in Non-patent Document 1.
As shown in FIG. 1, the random access procedure is constituted from the following four steps:
<Step 1>
Random Access Preamble (for Uplink) (Random Access Preamble on RACH in Uplink)
<Step 2>
Random Access Response (for Downlink) (Random Access Response Generated by MAC on DL-SCH (Down Link Shared Channel))
<Step 3>
First Scheduled Data Access (First Scheduled UL Transmission on UL-SCH (Up Link Shared Channel) (for Uplink)
<Step 4>
Data Access (for Downlink) (Contention Resolution on DL)
Details of the random access procedure and a general transmission and reception operation in a base station will be described, focusing on an uplink operation.
FIG. 2 illustrates a PRACH (Physical Random Access Channel: physical random access channel) preamble format (random access preamble format) used for initial access of a terminal in an LTE uplink. FIG. 2 corresponds to the figure on page 33 in Non-patent Document 2 (FIG. 5.7.1-1 Random access preamble format). The random access preamble is constituted from a CP (Cyclic Prefix) portion of 0.1 ms length (=TCP) and a Preamble Sequence portion of 0.8 ms length (TSEQ).
In the LTE, a frequency and time resource for the random access preamble in FIG. 2 is provided in advance for step 1 in FIG. 1. The terminal transmits the random access preamble in step 1, using the frequency and time resource specified for random access when the terminal undergoes a transition from the idle state to the connection state.
FIG. 4 illustrates, as a prototype example (reference case), a general configuration of a PRACH reception processing unit in a receiver on a base station side. Referring to FIG. 4, the PRACH reception processing unit includes a cyclic prefix removal unit 11, a DFT (Discrete Fourier Transform: discrete Fourier transform) unit 12, a subcarrier demapping unit 13, a preamble signal multiplication unit 14, an IDFT (Inverse Discrete Fourier Transform: inverse discrete Fourier transform) unit 15, and a maximum path detection unit 16.
In the reception processing unit in FIG. 4, cyclic prefixes are removed from a received signal received, using the frequency and time resource for the random access, by the cyclic prefix removal unit 11.
Next, the DFT unit 12 performs DFT on a signal having the cyclic prefixes removed. The subcarrier demapping unit 13 performs subcarrier demapping on a signal after the DFT (in a frequency domain) to extract a signal corresponding to a frequency resource specified for the random access.
The preamble signal multiplication unit 14 multiplies a signal subjected to subcarrier demapping by the subcarrier demapping unit 13 with a complex conjugate of a transmission preamble signal.
The IDFT unit 15 performs IDFT on the output signal of the preamble signal multiplication unit 14. The maximum path detection unit 16 calculates power per sample of the signal after the IDFT (in a time domain). The calculated power per sample is referred to as a “PRACH correlation value”.
The maximum path detection unit 16 determines peak (maximum) power of the PRACH correlation value. If the peak (maximum) power is equal to or larger than a preset threshold, the base station regards that the terminal has transmitted the preamble. The base station transmits the random access response in FIG. 1 by a downlink.
When the terminal not shown receives the random access response (in step 2 in FIG. 1) from the base station, the terminal performs data transmission, using a PUSCH (Physical Uplink Shared Channel: physical uplink shared channel) in the first scheduled data access (in step 3 in FIG. 1).
FIG. 3 illustrates a PUSCH subframe (Subframe) format. Duration of one subframe is 1 ms. The one subframe includes a 14 number of DFT-Spread-OFDM symbols and a CP (Cyclic Prefix: cyclic prefix) associated with each of the 14 number of DFT-Spread-OFDM symbols.
Third and tenth symbols from a 0th symbol (symbol on the left end of the 14 symbols) are referred to as reference symbols (Reference Symbols) (indicated by RSs). A known sequence is transmitted to and from both of a receiving side and a transmitting side as the reference symbols, and is used for channel estimation and frequency offset estimation for demodulating data on the receiving side. 12 symbols of the 0th, first, second, fourth, fifth, sixth, seventh, eighth, ninth, eleventh, twelfth, and thirteenth symbols (each indicated by D) other than the reference symbols are used for data transmission. The duration of a former-half slot (Slot #0) of the subframe is set to be 0.5 ms, and the duration of a latter-half slot (Slot #1) is set to be 0.5 ms. The duration between the reference symbols (RSs) in the former-half and the latter-half slots is set to be 0.5 ms.
On receipt of an uplink signal by the base station in step 3 in FIG. 1, the base station executes downlink data transmission in response to the uplink signal, in step 4 in FIG. 1.
Next, general PUSCH reception processing in step 3 in FIG. 1 will be described. FIG. 5 illustrates, as a prototype example (reference case), a general configuration of a PUSCH reception processing unit of the receiver on the side of the base station. The reception processing unit includes a cyclic prefix removal unit 21, DFT units 22-1 and 22-2, subcarrier demapping units 23-1 and 23-2, a reference signal multiplication unit 24, a channel estimation unit 25, a frequency offset estimation unit 26, a data equalization unit 27, and a demodulation unit 28.
The cyclic prefix removal unit 21 removes cyclic prefixes from a received PUSCH signal to divide the resulting signal into a data signal and reference symbols.
The DFT units 22-1 and 22-2 perform DFT on the data signal and the reference symbols received and obtained by the division. The subcarrier demapping units 23-1 and 23-2 performs subcarrier demapping on the signal after the DFT (in the frequency domain) to extract a frequency domain signal allocated to the user.
The reference signal multiplication unit 24A multiplexes a subcarrier demapped reference symbol with a complex conjugate of a transmitted reference symbol. Then, the channel estimation unit 25 obtains a channel estimation value.
Using an obtained channel estimation value, the frequency offset estimation unit 26 estimates a frequency offset amount.
Next, the obtained channel estimation value and a data signal after the subcarrier demapping are supplied to the data equalization unit 27. The data equalization unit 27 equalizes the frequency domain of the data signal.
Finally, the demodulation unit 28 converts the signal equalized in the frequency domain by the data equalization unit 27 into a time domain signal. Further, the demodulation unit 28 performs offset compensation on the signal converted into the time domain signal, using the frequency offset amount estimated by the frequency offset estimation unit 26.
A general example of the frequency offset compensation performed on the signal converted into the time domain signal is given by the following expression (1):sdem,comp(k)=sdem(k)·exp(−j2πk·Δf·ΔT); k=0, 1, 2,  (1)
where Sdem(k) (k=0, 1, 2, . . . ) is a complex signal (before the frequency offset compensation) converted into the time domain signal by the demodulation unit 28.
Sdem,comp(k) (k=0, 1, 2, . . . ) is a signal (after the frequency offset compensation) obtained by the frequency offset compensation performed on the complex signal converted into the time domain signal by the demodulation unit 28.
Δf[Hz] is a frequency offset estimation value estimated by the frequency offset estimation unit 26.
ΔT[s] is the duration of one sample of the demodulated signal converted into the time domain signal by the demodulation unit 28.
The frequency offset amount during initial access is obtained by taking correlation between complex channel estimation values of two slots obtained from the reference symbols (RSs) of the former-half and latter-half slots of the PUSCH signal, and by further obtaining an argument of the complex correlation value. This process will be described below in detail.
First, it is assumed that a complex channel estimation value as follows is obtained for each subcarrier allocated to the user:H(s,k)  (2)
where s is a slot number in one subframe, and s=0, 1 (0: former-half slot, 1: latter-half slot) (refer to FIG. 3).
k is a sub carrier number, and k=0, 1, . . . , N−1 (where N indicates the number of subcarriers allocated to the user).
The frequency offset amount Δf[Hz] is estimated as shown in the following Expressions (3) and (4), using the channel estimation values of two slots.
A correlation value R between the channel estimation values (H(s=0, k) and H(s=1, k)) of the two slots obtained from the received PUSH signal is calculated according to the following Expression (3):
                    R        =                              ∑                          k              =              0                                      N              -              1                                ⁢                                                    H                ⁡                                  (                                                            s                      =                      0                                        ,                    k                                    )                                            *                        ⁢                                                  ⁢            •            ⁢                                                  ⁢                          H              ⁡                              (                                                      s                    =                    1                                    ,                  k                                )                                                                        (        3        )            
where * indicates a complex conjugate.
      ∑          k      =      0              N      -      1        ⁢        
indicates a total sum of H (s=0, k)*H(s=1, k) when k ranges from 0 to N−1 (N being the number of subcarriers allocated to the user).
The frequency offset estimation value Δf [Hz] is calculated from the correlation value R given by the Expression (3), using the following Expression (4):
                              Δ          ⁢                                          ⁢                      f            ⁡                          [              Hz              ]                                      =                                                            arg                ⁢                R                            ⁢                              ⌊                radian                ⌋                                                    2              ⁢              π                                ×                      1                                          T                S                            ⁢                              ⌊                s                ⌋                                              ⁢                                          ⁢                      (                                          -                π                            ≤                              arg                ⁢                R                            <              π                        )                                              (        4        )            
where Ts[s] indicates the duration [unit: s (second)] between the two reference symbols (RSs) in the former-half and latter-half slots of the PUSCH signal, and
argR indicates an argument [unit: radian (radian)] of the complex correlation value R between the channel estimation values, and ranges from −π to +π.
Estimation of the frequency offset amount Δf, using the Expression (4) utilizes the fact that the argument argR of the complex correlation value R between the channel estimation values obtained from the PUSCH signal is given by:2π·Δf·Ts[radian]  (5)
In the LTE, it is defined that Ts=0.5 [ms] (=0.5×10−3) (refer to FIG. 3). Thus, the above Expression (4) becomes as shown in the following Expression (6):
                                                                        Δ                ⁢                                                                  ⁢                                  f                  ⁡                                      [                    Hz                    ]                                                              =                            ⁢                                                                                          arg                      ⁢                      R                                        ⁡                                          [                      radian                      ]                                                                            2                    ⁢                    π                                                  ×                                  1                                      0.5                    ×                                          10                                              -                        3                                                                                                                                                                    =                            ⁢                              1000                ×                                                                            arg                      ⁢                      R                                        ⁡                                          [                      radian                      ]                                                        π                                ⁢                                  (                                                            -                      π                                        ≤                                          arg                      ⁢                      R                                        <                    π                                    )                                                                                        (        6        )            
In case the abovementioned estimation method is used, an estimable frequency offset range [Hz] is uniquely determined by the following expression (7), based on the duration Ts [second] between the reference symbols (RSs) of the two slots (former-half and latter-half slots) to be correlated. The relational expression (7) is derived from the expression (4) of Δf=(argR)/(2πTs), where argR is in a relationship of −π≦argR<.
                                          -                          1              2                                ·                      1                          T              S                                      ≤                  ESTIMABLE          ⁢                                          ⁢          FREQUENCY          ⁢                                          ⁢          OFFSET          ⁢                                          ⁢                      RANGE            ⁢                                                  [            Hz            ]                          <                              1                          2              ⁢                                                                            ·                      1                          T              S                                                          (        7        )            
In the PUSCH subframe format specifications of the LTE (where Ts=0.5 [ms] (=0.5×10−3)), the estimable frequency offset range becomes from −1000 Hz to 1000 Hz based on the expression (7).
Patent Document 1 describes configurations of a control signal and a data signal (including a preamble transferred on a PRACH and a reference signal included in a frame on a PUSCH) on an air interface between a base station apparatus and a terminal apparatus, the terminal apparatus, and the base station apparatus in an LTE communication system. Patent Document 2 discloses an apparatus for quickly and correctly detecting a preamble code and a method (of estimating an integer carrier frequency offset of a target base station by detecting a preamble index of the target base station to select a target cell) in an environment where there is a carrier frequency offset. Patent Document 3 discloses a configuration or the like for determining a first frequency offset of a received signal sequence, using first and second preambles.    [Patent Document 1]    JP Patent Kokai Publication No. JP2008-136172A    [Patent Document 2]    JP Patent Kokai Publication No. JP2008-236744A    [Patent Document 3]    JP Patent Kohyou Publication No. JP2002-518880A    [Non-patent Document 1]    3GPP TS 36.300 V8.9.0 (2009-06) 101.5 Random Access Procedure    [Non-patent Document 2]    3GPP TS 36.211 V8.7.0 (2009-05) 5.7 Physical Random Access Channel    [Non-patent Document 3]    3GPP TS36.104 V9.0.0 (2009-06) B.3 High Speed Train Condition